1. Field of the Invention
This invention relates to an ion selective electrode apparatus having an earth electrode for reducing noise leaked to the ion selective electrode, and, more particularly, to an earth electrode for an ion selective apparatus of the flowthrough type.
2. Discussion of the Prior Art
Ion selective electrodes combined to constitute an ion selective electrode apparatus are generally used to electrochemically detect the activity of ions like sodium, potassium cation or chloride anion in blood. The concentrations of these ions in blood of healthy or normal human beings lie in respective narrow ranges. In other words, an abnormal concentration of these ions indicates disease or malfunction of body organs. Actually, they provide diagnostic proof or information for determining, for example, the presence of kidney troubles, myocardial infarction, or manic depressive psychosis. The measurement of activity of ions in the blood indicates the concentration of the ions, for the concentration of ions is proportional to their activity.
An ion selective electrode apparatus usually comprises an ion selective electrode and a reference electrode. The ion selective electrode includes an ion selective membrane on the surface of it. The ion selective membrane has holes as large as a specific ion and has an opposite polarity to that of the ion. The holes in the membrane tend to capture the specific ions for a longer time than any other ions. The reference electrode, on the other hand, keeps a constant voltage difference across the solution so that the specific ions dissolve. When both the ion selective and reference electrodes are immersed in the solution, the potential of the ion selective electrode is different from that of the reference electrode. As a result, the electrodes induce a potential E related to the activity A of a specific ionic species present in the solution, as shown by the Nernst equation: EQU E=E.sub.0 .+-.2.303 (RT/ZF) log A
Wherein R, T, Z, F, and E.sub.0 represent the gas constant, the temperature of the solution expressed in terms of the absolute temperature, the ionic valence of the specific ion species, the Faradic constant, and the potential of the reference electrode, respectively. The (+) sign is employed when A is induced by cationic activity, and the (-) sign is employed when A is induced by anionic activity. As a result, the activity of the specific ion species can be readily calculated according to the Nernst equation.
An ion selective electrode apparatus of the flowthrough type is available in the prior art for an automatic chemical analyzer which automatically analyzes many samples such as patient serums according to a number of items. Such an ion selective electrode is taught in U.S. Pat. No. 4,533,457 issued to Watanabe. The electrode apparatus taught by Watanabe includes a tube of substantial size where the samples flow. The inner peripheral surface includes a plurality of ion selective electrodes and a reference electrode, and when the samples flowthrough the tube, the activities of the several kinds of ions are simultaneously obtained from the potentials between the respective electrodes and the reference electrode. This type of selective electrode apparatus is compact and is useful for measuring many samples for a number of analysis items in a short period of time. However, in this conventional electrode apparatus, electrical noise may leak to the electrode via the flowthrough.
Since an ion selective electrode has a high input impedance, an electrical noise impinged on it adversely affects the measurement of the activity of an ion. Therefore, the electrical noise introduced via the flowthrough makes the measurement of activity of a certain ion less accurate.